A memory cell, such as a dynamic random access memory (DRAM) cell, typically includes a charge storage capacitor coupled to an access device, such as a Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET). The MOSFET functions to apply or remove charge on the capacitor, thus affecting a logical state defined by the stored charge. The conditions of DRAM operation, such as operating voltage, leakage rate and refresh rate, will, in general, mandate that a certain minimum charge be stored by the capacitor.
Capacitors include two conductors, such as parallel metal or polysilicon plates, which act as electrodes. The electrodes are insulated from each other by a dielectric material. One type of capacitor used in DRAM cells is a metal-insulator-metal (MIM) capacitor. The dielectric constant, k, of the dielectric material (i.e., insulator material) in the capacitor is a crucial element for mass-producing DRAM cells. For example, a 3×nm DRAM cell or larger requires a dielectric material having a dielectric constant of at least about 55 in order to achieve the desired capacitance in capacitor.
Crystalline dielectric materials tend to have a higher dielectric constant than amorphous dielectric materials. For example, rutile TiO2 has a dielectric constant of about 170 along the c-axis of the crystal structure and about 90 along the a-axis of the crystal structure, while amorphous TiO2 has a dielectric constant of about 30. However, because of the crystal formation of the rutile TiO2, the rutile TiO2 has a very rough surface, which results in an increased current leakage. In conventional semiconductor structures, TiO2 grown by atomic layer deposition (ALD) has an amorphous structure and a dielectric constant of about 30.